In recent years, in order to meet the demand for ever smaller, more functional portable devices, studies on further improving the energy density of the secondary batteries to be installed on such devices have been conducted vigorously.
A lithium-ion secondary battery having a lithium transition metal oxide positive electrode and a graphitic carbon negative electrode can be a typical example of high energy density secondary batteries. And, aiming for the realization of much higher energy density, the use of metal materials such as Al, Sn and Si, capable of alloying with Li, as a negative electrode is being considered.
However, these materials suffer significant changes in volume due to alloying; hence, the secondary batteries using these materials as a negative electrode are poor in cycle performance and insufficient for practical use. Some proposals for solving these problems have been made: for example, the use of an alloy consisting of Sn and Ni, the former is capable of alloying with Li and the latter is not (e.g. Japanese Provisional Publications No. 143700 of 2001 and No. 162823 of 1998.) These proposals are designed to achieve a secondary battery with high energy density and excellent cycle performance by using the properties of Sn and Ni, that is, Sn for the attainment of high energy density and Ni, which does not alloy with Li, for the maintenance of the framework structure of alloy and for the prevention of alloy degradation due to volume change.
The inventor of the present invention, too, has conducted studies on a negative electrode which is capable of achieving high energy density by the use of Sn. And, as a result, he has found that it is impossible to attain sufficient cycle performance only by mixing Sn and Ni and perform reversible charge/discharge by using a complete alloy of Sn and Ni, for example, Sn4Ni3 alloy, and that when an alloy having Sn phase and Sn4Ni3 phase is used, it is possible to realize satisfactory cycle performance and high energy density.
It is an object of the present invention to provide a non-aqueous electrolyte secondary battery having high energy density and satisfactory cycle performance by using an alloy comprising Ni and Sn as a negative active material.